azurerm_application_gateway - correctly poopulat the identity block

[magodo]

This PR fixes the helper flatten/expand functionality of the userAssigned identity in the identity package. Meanwhile, also fixes the only place that the prior errorneous user assigned identity is used, namely the azurerm_application_gateway data source.

Test Result

💤 TF_ACC=1 go test -v -timeout=8h ./azurerm/internal/services/network -run="TestAccDataSourceAppGateway_userAssignedIdentity"


2021/06/16 14:52:23 [DEBUG] not using binary driver name, it's no longer needed
2021/06/16 14:52:24 [DEBUG] not using binary driver name, it's no longer needed
=== RUN   TestAccDataSourceAppGateway_userAssignedIdentity
=== PAUSE TestAccDataSourceAppGateway_userAssignedIdentity
=== CONT  TestAccDataSourceAppGateway_userAssignedIdentity
--- PASS: TestAccDataSourceAppGateway_userAssignedIdentity (996.53s)
PASS
ok      github.com/terraform-providers/terraform-provider-azurerm/azurerm/internal/services/network     996.907s

[tombuildsstuff]

LGTM 👍

[github-actions]

This functionality has been released in v2.64.0 of the Terraform Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

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[github-actions]

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